Conventional target object detection devices transmit radio waves for detection and detect target objects based on echo signals of the transmitted radio waves. As one kind of such target object detection devices, a radar equipped in a ship and for detecting a target object (e.g., another ship on the sea) and a land by transmitting and receiving electromagnetic waves by an antenna has been known. With the radar, unnecessary components caused by, for example, a sea surface reflection echo (sea clutter) is suppressed so that the target objects (e.g., the other ship and land) are easily distinguished. Here, a threshold is set to determine whether the echo signal is an unnecessary component, and determine a suppressing amount of the echo signal.
As a conventional threshold setting method, a method so called CFAR processing (Constant False Alarm Rate processing) has been known, which calculates a moving average using a group of echo signals in a distance direction, in other words, in a direction away from an antenna position (see Sekine Matsuo, “Radar Signal Processing Techniques” The Institute of Electronics, Information and Communication Engineers, Sep. 20, 1991).
However, sea surface reflection echoes have a characteristic in which a level difference generally becomes larger as a distance from a radar apparatus (a ship) is shorter. Therefore, even when the CFAR processing is used, a threshold level will be set as an average value of levels of the sea surface reflection echoes. Therefore, the sea surface reflection echoes cannot be suppressed appropriately by using such a threshold. Particularly, when the sea surface reflection echo has a high signal level, the suppressing effect is low.